Generally, in a High-Voltage Direct Current (HVDC) system, AC power, produced in a power plant, is converted into DC power and then transmitted, and the transmitted DC power is converted into AC power on a power reception side and then supplied to a load. The HVDC system may transmit power effectively and economically by increasing voltage, and is advantageous in that it allows interconnection between asynchronous grids and efficient power transmission over long distances.
In the HVDC system, a Modular Multilevel Converter (MMC) is used for power transmission and compensation for reactive power. Such an MMC includes multiple sub-modules, which are connected in series with each other. Because the sub-modules are very important components in the MMC, a power supply apparatus for stably supplying power to the sub-modules in various environments is required.
FIG. 1 is an equivalent circuit diagram of an MMC, and FIG. 2 is a circuit diagram of a conventional power supply apparatus for the sub-modules of an MMC. As is well known, an MMC consists of one or more phase modules 1, and each of the phase modules includes multiple sub-modules 10, which are connected in series with each other. Also, each of the phase modules 1 is connected with a positive (+) DC voltage bus P and a negative (−) DC voltage bus N. The input voltage between the P and N buses is input to the sub-module 10 through connection terminals X1 and X2.
In order to supply power necessary for the operation of sub-modules, a power supply apparatus for sub-modules of an MMC converts a high voltage (about 2 to 3 kV) between the P and N buses into a low voltage (about 300 to 600V) and supplies the low voltage to the sub-modules. To this end, in the conventional power supply apparatus 20, while an input voltage between the P and N buses of the MMC increases from OV to a high voltage (for example, 3 kV), the voltage Vdc is stored in a capacitor 21. That is, while the voltage Vdc, which is the input voltage between the P and N buses, increases from 0V to 1000V, the clamping voltage Vzd of a Zener diode (ZD) 23 is output to a controller 24, and when the clamping voltage Vzd is input to the controller 24, the controller 24 enables current to be supplied to a transformer 26 by turning on a switch. Accordingly, when a voltage Pcon, output from the secondary winding of the transformer 26, is applied to the controller 24, the controller 24 operates the power supply apparatus 20.
In this case, the power supply apparatus 20 starts to operate even if the input voltage Vdc is lower than the rated voltage of the power supply apparatus 20, but the operation is interrupted without producing normal output due to the low input voltage. Here, the operation is repeatedly resumed and interrupted while the input voltage increases, and such repetition finishes when the input voltage reaches the rated voltage. When the input voltage reaches the rated voltage and the output of the power supply apparatus 20 becomes normal, the voltage Pcon output from the secondary winding of the transformer 26 supplies power to the controller 24.
As described above, in the conventional art, the power supply apparatus 20 starts to operate even if the input voltage is lower than the rated voltage at the beginning, but the apparatus cannot operate normally. Also, because current continuously flows to a resistor 22 and a Zener diode 23 in the state in which the input voltage reaches a high-voltage section while the input voltage continuously increases, heat is generated in these elements 22 and 23, which generates energy loss.
Therefore, in the technical field pertaining to a power supply apparatus for sub-modules of an MMC in connection with an HVDC system, the development of technology for a power supply apparatus that may eliminate unnecessary operation and reduce loss is required.